1. Field of the Invention
The present invention relates to an optical fiber characteristics measuring device and an optical fiber characteristics measuring method, particularly, a driving control for a laser diode.
This application is based on Japanese Patent Application No. 2000-93839, the contents of which are incorporated herein by reference.
2. Description of Related Art
FIG. 2 shows the structure of a conventional optical characteristics measuring device. In FIG. 2, a laser diode 102 emits a continuous light by a laser diode driving circuit 100, and the continuous light is branched into a reference light and a signal light in a first optical directional coupler 104. The signal light is modulated by light intensity modulation in an optical switch 106 so as to become a pulse light from the continuous light. In this state, since the continuous light and the pulse light are both required to have the same frequency, the optical switch 106 is used to carry out as the light intensity modulation.
In the optical switch 106, a bias voltage and drift must be controlled. Furthermore, an extinction ratio of the signal light is approximately 15 to 30 dB. If an extinction ratio is required which is larger than the above extinction ratio which results when only one optical switch is provided, several optical switches must be inserted in the device.
Therefore, in the conventional optical characteristics measuring device, if a large insertion loss (for example, 5 dB or larger) or a delayed response time of a control signal (for example, 3 ns or longer) is generated depending on the optical switch, a pulse light having a short pulse width (10 ns or shorter) cannot be generated. Furthermore, since the characteristics of the optical fiber depend on the polarization of light, then the polarization of light must be controlled.
The optical switch 106 modulates the continuous light into the pulse light, the pulse light is amplified by an optical fiber amplifier 108, and further the amplified pulse light is sent as a pulse signal to an optical fiber 116 to be measured via a second optical directional coupler 110. A backward scattered light, which is the signal light returned from the optical fiber 116, and the reference light are coupled in a third optical directional coupler 112, so that the backward scattered light having low optical power can be detected by a photoelectric transducer 114.
However, as described above, although it is necessary for the pulse light and the continuous light to have the same frequency (wavelength), it is difficult to manufacture or control the laser diodes so that each has exactly the same frequency. If the continuous light is emitted by a single laser diode and the pulse light and another continuous light are obtained from the continuous light, an optical switch and an optical fiber amplifier for pulse amplification are required. Furthermore, there are many difficult problems such as control of the bias voltage and drift which may be generated in the optical switch, and an improvement of the pulse amplification characteristics of the optical fiber amplifier.
Moreover, it is difficult to attain amplification of the pulse light by the optical fiber amplifier, especially, when the duty ratio is large. When the signal light is continuously input into the optical fiber amplifier, the pulse light is in a stable state; however, when the signal light is intermittently input into the optical fiber amplifier, that is, when the pulse period of the signal light is long, the pulse light is in an unstable state. In this state, energy is stored in the optical fiber amplifier and the stored energy is output all at once just when the signal light is input into the optical fiber amplifier. Then, the pulse light has a form like a sawtooth wave and becomes unstable. As the time when the signal light is not emitted between each pulse or when no signal light is input into the optical fiber amplifier becomes longer and longer, energy is stored in the optical fiber amplifier, and therefore, the duty ratio of the signal pulse light may be set as small as possible.
In light of the above-mentioned problems, an object of the present invention is to provide an optical fiber characteristics measuring device and an optical fiber characteristics measuring method, in which a pulse light and a continuous light, both having the same frequency, are obtained at the same time, and thereby the number of optical parts is reduced and control is simplified.
To achieve the above-mentioned object, according to the first aspect of the present invention, an optical fiber characteristics measuring device is provided, comprising: a pulse driving circuit for supplying a pulse driving current into a laser diode; a first optical directional coupler for branching the light output from the laser diode into a signal light and a reference light; a second optical directional coupler for sending the signal light to an optical fiber to be measured and for branching a returned light from the optical fiber into a backward scattered light and the signal light; a third optical directional coupler for incorporating the reference light output from the first optical directional coupler and the backward scattered light output from the second optical directional coupler and for coupling the backward scattered light onto the reference light; a photoelectric transducer for converting an optical signal output from the third optical directional coupler into an electric signal and for outputting the electric signal; and a bias circuit for supplying a continuous driving current to the laser diode at all times, wherein the frequency of the backward scattered light is detected based on the electric signal.
According to the above device, since the bias circuit which supplies a continuous driving current to the laser diode at all times is provided, the laser diode itself is always emitting. Therefore, the pulse light generated by the laser diode pulse driving circuit and the continuous light generated from the bias circuit are obtained at the same time. Furthermore, since the optical switch and the optical fiber amplifier for the pulse amplification are not required, control of the bias voltage and the drift of the optical switch are not required. Therefore, in the optical fiber characteristics measuring device in which it is required that the pulse light and the continuous light each have the same frequency, the optical structures are simplified. Furthermore, since the laser diode itself always emits light, chirping is prevented.
Furthermore the above-mentioned optical fiber characteristics measuring device may further comprise a delay circuit. The delay circuit is provided between the first optical directional coupler and the second optical directional coupler, and delays the time at which the signal light output from the first optical directional coupler arrives at the third optical directional coupler so as to be later than a timing that the reference light output from the first optical directional coupler arrives at the photoelectric transducer.
According to the above device, since the delay circuit is provided, the detection accuracy when detecting the frequency of the backward scattered light output from the optical fiber to be measured is improved.
Furthermore, the above-mentioned optical fiber characteristics measuring device may further comprise an optical fiber amplifier. The optical fiber amplifier is provided between the first optical directional coupler and the third optical directional coupler in order to amplify the reference light.
According to the above device, since the optical fiber amplifier is provided, an amplified reference light, especially, an amplified continuous light is obtained, and therefore, even if the pulse light has a form like a sawtooth wave, the pulse light remains stable even though the pulse light in a conventional optical fiber characteristics measuring device becomes unstable when having the form of a sawtooth wave.
According to the second aspect of the present invention, an optical fiber characteristics measuring method is provided, comprising the steps of: supplying a pulse drive current into a laser diode; supplying a continuous driving current into the laser diode with the pulse drive current; branching light output from the laser diode into a signal light and a reference light; inputting the signal light into an optical fiber to be measured; coupling the reference light onto a backward scattered light returned from the optical fiber to form an optical signal, converting the optical signal into an electric signal; and detecting the frequency of the backward scattered light based on the electric signal.
According to the above method, since the continuous driving current is supplied in addition to the pulse drive current into the laser diode, the pulse light and the continuous light are obtained at the same time. Furthermore, since the optical switch and the optical fiber amplifier for the pulse amplification are not required, control of the bias voltage and the drift of the optical switch are not required. Therefore, in the optical fiber characteristics measuring device in which it is required that the pulse light and the continuous light each have the same frequency, the optical structures are simplified. Furthermore, since the laser diode itself always emits light, chirping is prevented.
According to the present invention, the pulse light and the continuous light, both having the same frequency, are obtained at the same time, and thereby, the number of optical parts in the optical fiber characteristics measuring device is reduced and control is simplified.